The present invention is directed to quasi-resonant (QR) systems. More particularly, the invention provides dual-mode QR systems and methods that are used for electronic devices. Merely by way of example, the invention has been applied to controllers of switch-mode power converters, such as flyback switch-mode power converters. But it would be recognized that the invention has a much broader range of applicability. For example, the present invention can be applied to devices other than integrated circuits. In another example, the present invention is applicable to any power conversion systems that use QR control.
The quasi-resonant (QR) technology has been widely used in power converters for medium and high power levels. These power converters often include various control topologies, such as the flyback topology. A conventional QR system may enable Zero-Voltage-Switching (ZVS), which is important for many high-power applications. Additionally, the conventional QR system can reduce the number of external devices by utilizing parasitic devices.
FIG. 1 is a simplified diagram showing a conventional flyback pulse-width-modulated (PWM) system with QR control. The flyback PWM system 100 with QR control includes an auxiliary winding to generate a demagnetization signal DEM for a QR controller. For example, the DEM signal indicates the demagnetization of a power transformer T as shown in FIG. 1. Additionally, the system 100 also includes one or more components for generating a DC input voltage Vin. The DC input voltage Vin may be a rectified line voltage or the output of a power-factor-correction (PFC) stage. For example, the PFC stage is placed between a diode bridge and a DC-to-DC converter for certain high-power applications. Such high-power applications may include flyback power converters with QR control and/or forward power converters with QR control.
As shown in FIG. 1, the flyback PWM system 100 is associated with a combination of Lleak and Cp. Lleak is the leakage part of the primary inductance Lm, and Cp is the parasitic capacitance at the drain of the MOSFET S1.
FIG. 2 is a simplified diagram showing operation mechanism of a conventional flyback pulse-width-modulated (PWM) system with QR control. For example, the conventional flyback pulse-width-modulated (PWM) system is the system 100.
As shown in FIG. 2, at t0, the PWM switching is enabled. The power MOSFET S1 (as shown in FIG. 1) is turned on. Consequently, the current of the primary inductor ramps up, and the power transformer T stores energy.
At t1, the current of the primary inductor ramps up to a value that is determined by the feedback. The power MOSFET S1 is turned off. The drain-to-source voltage Vds of the MOSFET S1 rises rapidly because of the transformer current. The peak value of the drain-to-source voltage Vds is determined by the leakage inductance Lleak, the DC input voltage Vin, and the reflected output voltage Vr. Vr is equal to N×Vout, as shown in FIG. 1.
At t2, the demagnetization of the leakage inductance Lleak is completed, and the primary inductance Lm (as shown in FIG. 1) begins to demagnetize.
At t3, the demagnetization of the primary inductance Lm ends, and the damping resonance starts. The resonance period equals 2×Tv, which is determined by Lm and Cp. As shown in FIG. 2, the resonance often generates one or more valleys. For example, the first valley occurs at t4, and the second valley occurs at t5.
At one of these valleys, a new PWM cycle is restarted by the QR controller. If the new PWM cycle is started at the first valley at t4, the QR controller operates in the QR mode. If the new PWM cycle is started at a subsequent valley, such as the second valley at t5, the QR controller operates in the QR foldback mode.
Since at these valleys, Vds equals zero or a local minimum, the efficiency of the system is improved. For example, the first valley at t4 often is selected because this valley corresponds to the smallest local minimum.
The conventional flyback PWM system 100 with QR control has the following characteristics in comparison with a conventional flyback PWM system with fixed frequency:
(a) Improved EMI performance. Without a PFC stage, the switching frequency of the system 100 can be modulated at twice the line frequency due to the ripple across the input bulk capacitor. The depth of the modulation also depends on the ripple magnitude. Hence the spectrum spreads over one or more frequency bands, rather than being concentrated on single frequency values. It is then possible to reduce the size and cost of the EMI filter.
(b) Improved power efficiency. For example, the system 100 can substantially achieve zero voltage switching (ZVS); therefore the power efficiency of the system 100 is improved.
(c) Inherent short circuit protection. For example, the conduction cycles of the power MOSFET are inhibited until the transformer is full demagnetized; hence the transformer saturation is not possible. In another example, during a short circuit, the demagnetization voltage is very low; hence the system operates at a low frequency with a small duty cycle. As a result, the power delivered by the converter is also low.
FIG. 3 is a simplified diagram showing a conventional flyback PWM system including a conventional QR controller. As shown in FIG. 3, the flyback system 300 includes a QR controller 310. The controller 310 includes a flip-flop block, a UVLO&POR block, a DEM block, a PWM generator block, and an LEB&OCP block. For example, the UVLO&POR block can provide power supply to a control IC, and the DEM block can detect demagnetization of the transformer T1 and trigger a new PWM cycle. The PWM generator block can control the peak primary current. The LEB&OCP block is used for leading edge blanking and over current protection. As shown in FIG. 3, the auxiliary winding can provide not only the power but also the DEM signal to IC.
The conventional techniques of flyback PWM systems with QR control may be costly and large in size. Hence it is highly desirable to improve techniques that are related to QR control.